The first two parts of this small series concentrated on acquiring and then powering a laser diode from a DVD/RW drive. In this post we get to the fun stuff - using the laser to cut and burn.

I tried the laser jig I described in the previous post on a range of different materials to see what effect it had on them. If you are going to try this for yourself make sure you adhere to the safety guidelines I outlined in the first post. As we are using the lasers heat to burn through materials there is going to smoke and (depending on the material) some of it can be toxic so make sure that any experiments you do are in a well ventilated area.

Foam and Plastic

This type of material is perfect for this laser - plastic has a low melting point and (as long as it's not reflective and doesn't contain any red pigment) the laser goes through it very rapidly. This is the only material I've tried that I could achieve a complete cut at any reasonable depth.

The image to the left shows the result after applying the laser to 2mm thick craft foam. I had some thin strips of various colours as well as rainbow coloured sheets - perfect for seeing what difference the colour of the material has. With foam the laser has an almost immediate effect and can be moved fairly rapidly (2 or 3 mm/sec) to continue the cut.

The foam does need to be a dark colour though and not reflect a lot of red light. If you look at the bottom of the image you will see that the cut comes to a dead stop at the boundary of the yellow section - this was not done on purpose. I was moving the laser at the same rate past that boundary but it was not providing enough energy to cut through the foam.

Any black plastic is another good candidate for use with the laser. It is easy to scour designs into thicker plastic and thinner plastic (like duct tape) can be cut through directly. I haven't provided any photos of these though - the duct tape is the most interesting but the cuts are very thin and I was unable to capture a good photograph that showed it clearly.

Wood and Paper

I didn't have a lot of samples of paper to test with apart from the normal white photocopier paper - it turns out that matte black paper is very difficult to find. Everything I could find had a glossy finish which simply reflected too much of the laser light to allow enough heat to develop to mark or cut.

Although I could not start a cut on plain white paper I found that if I started at an already darkened area (a pencil mark or a printed character) I could initiate a cut and then continue it into the lighter coloured areas. The image to the right shows a cut started on a laser printed character and continued into the main part of the page (again at 2 to 3 mm/sec). Once the cut started the surrounding area becomes dark enough to absorb enough energy to continue the cut as you move the beam.

This turned out to be a common theme with lighter coloured materials, my experiments with wood had very similar results. I collected a range of different woods from around the lab - I wound up with some balsa, some pine, paddle pop sticks and MDF. Obviously you will not be able to cut through wood but you can make a deep enough burn to permanently mark it.

With lighter coloured woods it was very difficult (if not impossible) to start the burning process, as with the white paper I had to darken an area of the wood first (I just used a pencil) and, once initiated, the burn would continue without interruption.

The denser the wood the slower the laser can advance to continue the burn and the longer it takes to initiate it. Even though MDF is far darker than pine or balsa it still need assistance to get started and the movement rate of the laser was reduced to less than 1mm/sec in order to reliably continue it. This was the rate I used by hand and was just enough to continue a surface mark, if you want to make deep burns the rate would need to be even slower.

PCB Etching

You cannot cut through even the thin layer of copper on a PCB with a 400mW laser, using it to facilitate PCB etching is a multi-step process:

We need to coat the copper side of the PCB with a material that can be burnt away with the the laser.

Burn away the coating to expose the copper that we want to get rid of.

Etch the board as normal and then remove the excess coating.

The results (when using a commercial laser etcher) can be very impressive (as seen in the image to the left). What I wanted to find out was how well I could replicate that with a lower powered laser.

The original post I came across described using a thin layer of spray paint to provide the protective surface that will be burnt away. I tried that with a matte black enamel spray paint and was unable to make any mark in the surface at all.

On the assumption that a general use paint would contain heat resistant material I decided to experiment with some different types of paint and ink (as shown in the image to the right). Unfortunately I made no progress with any of these either, I was not able to even leave a mark in the coating let alone burn it away to expose the surface beneath. My current working theory is that the heat dissipates too quickly (copper is a good conductor after all) before the covering material can heat up enough to ablate.

Summary

Given that my original goal for these experiments was to be able to use the laser to help create PCBs the results are a bit disappointing. It was certainly fun to play with (lasers!) but in terms of utility it is a bit limiting - at the power levels available at least.

As an alternative tool head for a CNC machine or a 3D printer it would be useful, adding the ability to cut foam and paper or permanently mark wood and plastic. There is a reasonable investment of time and effort required to get it working though so you would have to justify that against how often you would use it.

As a tool for preparing PCBs a 300mW or 400mW laser is simply not going to do the job unfortunately.